NEW YORK—It’s the largest and most valuable blue diamond to ever come to auction. And, it’s a recent discovery. The more than 15-carat De Beers Cullinan Blue diamond was mined in 2021 at South Africa’s Cullinan Mine, one of the few sources in the world for extremely rare blue diamonds. It will be offered in a single-lot auction at Sotheby’s Hong Kong this April and is estimated to be valued in excess of $48 million. Sotheby’s Senior Vice President and Sales Director for Jewelry Frank Everett said the jewel is remarkable on many levels. “It’s rare because of the size. It’s over 15 carats. It’s a vivid blue. It’s internally flawless. And really one of the most rare aspects of it is the cut,” he said. “I think we’re going to see tremendous interest in this stone when it finally comes to auction,” he said. “The market is very strong …